1. Field of the Invention
The present invention relates to dye-donor elements for use in thermal dye sublimation transfer methods, the dye-donor elements comprising a subbing layer that improves the adhesion between the polymeric support and the dye layer and/or between the polymeric support and the heat-resistant layer.
2. Background of the Invention
Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors so as to transfer dye from the selectively heated regions of the dye-donor element to the receiver sheet, thus forming a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer that contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer.
During the printing operation the thin support softens when heated and thereby may stick to the thermal printing head, thus causing malfunction of the printing apparatus and reduction in image quality. To avoid this and to facilitate passage of the dye-donor element under the thermal printing head the rear side of the support (side opposite to that of the dye layer) may typically be provided with a heat-resistant layer. An adhesive or subbing layer layer is usually provided between the support and the heat-resistant layer. Such subbing layers have been disclosed in e.g. EP 138,483, EP 227,090, EP 407,220, EP 433496, U.S. Pat. Nos. 4,572,860, 4,717,711, 4,559,273, 4,695,288, 4,727,057, 4,737,486, 4,965,239, 4,753,921, 4,895,830, 4,929,592, 4,748,150, 4,965,238, and 4,965,241.
Thin subbing layers of the polyesters disclosed in EP 407,220 and U.S. Pat. No. 4,727,057 tend to solubilize when they are covered with the heat-resistant layer.
Provided they are used in the form of thick subbing layers, random linear copolyesters such as those described in U.S. Pat. No. 4,727,057 and in EP 407,220, exhibit good adhesion properties in respect of the heat-resistant layer.
The use of thick polyester subbing layers results, however, in a lower degree of stability of the dye-donor element in rolled form and in a lower thermostability of the heat-resistant layer owing to mixing of a part of the polyester with the polymers present in the heat-resistant layer.